An industrial robot is generally configured such that an arm including a plurality of joints is connected to a robot main body and a tool is provided at a tip of the arm. A servo motor is attached to the robot main body, and a position detecting member is attached to the servo motor. A servo control section controls a tool position, arm position, arm angle and the like based on present position information of the tool detected by the position detecting member.
As a method for monitoring an operation of the robot, for example, there has been disclosed a conventional technique of an invention of the patent application by the applicant of the subject application, in other words, a method for detecting an abnormality by comparing a difference between an instruction position received from a position control section and a present position input from the position detecting member with a threshold value corresponding to an instruction speed (for example, see Patent document 1).
Moreover, by developing the above conventional technique, there has been disclosed a method for detecting an abnormality with higher accuracy by modeling an operation of a robot arm so as to suppose an actual position based on the instruction position from the position control section to compare the supposed actual position with the present position input from the position detecting member (for example, see Patent documents 2 and 3).    Patent document 1: Japanese Patent Application Laid-Open Publication No. Hei 4-67209    Patent document 2: Japanese Patent Application Laid-Open Publication No. Hei 10-315173    Patent document 3: Japanese Patent Application Laid-Open Publication No. 2004-181543